One of the most important symptoms is post-exertional neuroimmune exhaustion. It means the other symptoms—pain, debilitating exhaustion, etc., get worse after physical or mental exertion and require an extended recovery period.
A study published last month may shed some light on why post-exertional neuroimmune exhaustion exists:
The cells from ME patients produced on average 20 times as much acid when exercised, suggesting an underlying cause for the aching muscles that patients often experience as soon as they begin to exercise.
(Interpret "exercise" to mean any physical exertion, not exercise like pushups.)
Here's hoping for more progress by this time next year.